In recent years, along with the rapid development of medicine, pharmacy and biological tissue engineering, the demand for materials featuring good biocompatibility, biological safety and biodegradability is rapidly growing throughout the world. Biodegradable polylactic acid has been extensively applied in modern pharmaceutical science (controlled-released and targeted drug carriers), biological tissue engineering science (surgical sutures, bone screws, fracture splints, meniscus-repairing materials and devices, cells and bioactive species scaffold materials) and the like. It is required that the polymers used in the field of pharmacy and medicine should not contain any metal and toxic ingredient. Currently, the production of commercially available polylactic acid is mainly performed as follows: 1. ring-opening polymerization of lactide catalyzed by stannous octoate; 2. polycondensation of lactic acid catalyzed by stannous chloride. Although both methods can synthesize a polymer with desired molecular weight, the tin salt catalyst cannot be completely removed from the polymer after polymerization. In recent years, by the studies of scientists worldwide, it has been definitely proved that both stannous octoate and stannous chloride exhibit cytotoxicity. Therefore, the safety issue of polylactic acid for pharmaceutical and medical materials for human, synthesized by using stannous octoate or stannous chloride as catalyst, has been extensively questioned by scientists all over the world. The exploration for efficient, non-toxic, and metal-free catalysts for synthesizing metal-free, non-toxic, and highly biologically safe polylactic acid materials for medical and pharmaceutical applications has become the urgent issue proposed by scientists in the field of biomaterial science throughout the world.
Based on the invention of domestic scholars such as Professor LI Hong and the co-workers, wherein the non-toxic, metal-free and highly biologically safe polylactic acid was successively synthesized via direct polycondensation catalyzed by creatinine (Mw=1.5˜3.0×104) (Chinese Patent Application: 201110059090.8), the present invention employs bionic organic catalyst firstly developed, creatinine-guanidinium chloride, to provide a process for synthesizing polylactic acid with a high molecular weight for medical and pharmaceutical purposes via direct polycondensation of lactic acid.